Aerodynamic drag is created whenever a ground vehicle travels at speed. Aerodynamic drag is comprised of two principle components—skin friction drag and form drag. The skin friction drag is a consequence of the air's viscosity and the “no-slip” condition that exists at the vehicle's surface. Even for the flow of fluids of very low viscosity—air for example—there is a region where the effects of the fluid's viscosity dominates. This area is called the boundary layer. FIG. 1 shows the flow of a freestream of air 1, over a stationary wall 2. As a result of the no-slip condition 3 at the wall, the velocity at the wall 2 is zero. The layer of fluid that corresponds to the distance from the no-slip condition 3 at the wall to the re-establishment of the freestream velocity is called the boundary layer 4.
Within the boundary layer, adjacent layers of fluid will be traveling at different velocities. The different velocities are the result of shearing stresses that are produced in the fluid. The shearing stresses are produced by the fluid's viscosity. Outside the boundary layer—in the freestream—all fluid will be traveling at the same speed and the effect of the fluid's viscosity will be negligible. For the specific case of a ground vehicle, the no-slip condition dictates that the air immediately adjacent to the vehicle will travel at the same speed as the vehicle. As discussed above, outside the boundary layer the air will be traveling at the air's free stream velocity. For the purposes of this discussion it will be assumed that the vehicle is stationary and the air is moving at the vehicle's velocity. This convention does not change the physics involved but it does make for an easier situation to describe. The shearing stresses in the boundary layer that produce the velocity gradient across the boundary layer also act as a retarding force on the vehicle's motion—i.e., drag. This component of drag is called skin friction drag.
For high speed, streamlined vehicles like jet aircraft, skin friction drag is the major contributor to aerodynamic drag. FIG. 2 shows the flow of air over a symmetric airfoil, 5, with chord, 6. A freestream flow of air, 1, is introduced and directed over the airfoil, 5. Because of the streamlined shape of the airfoil, 5—only gradual changes in surface profile exist—the flow is largely able to proceed in a direction of decreasing pressure. As a result the boundary layer flow remains “attached” to the airfoil, 5. The attached flow is shown as 7.
However, for ground vehicles—particularly for bluff, i.e., non-streamlined bodies like tractor trailer trucks—the major contributor to drag is form drag. FIG. 3 shows the flow of a freestream of air over a typical automobile 8. Form drag is created by separation of the boundary layer from the vehicle. As described above, the shearing stresses present in the boundary layer cause the air in the boundary layer to slow down relative to the speed of the vehicle. As a result of its reduced energy content the boundary layer of air may no longer remain attached to the vehicle, particularly when the air is forced to flow into an area of increasing pressure. Areas of increasing pressure are produced by surfaces with a large radius of curvature or a geometric discontinuity. Areas of increasing pressure can be reduced by vehicle streamlining but there are practical limits on what can be achieved with streamlining commercial vehicles. In the flow of air around conventional ground vehicles areas of increasing pressure cannot be avoided. In places of increasing pressure the air can begin to recirculate or flow in the opposite direction.
This flow in areas of increasing pressure is seen in FIG. 3 as the air tries to flow along the vehicle's rear deck 9. The large radius of curvature of the deck leads to a large increase in area for the air to flow through and the air will slow down as a result. This is the same phenomenon that occurs in a diffuser and as with a diffuser the flow along the rear deck 9 leads to an increase in pressure. If the increase in pressure is sufficiently large the boundary layer flow of air around the vehicle can separate from the vehicle. Boundary layer separation is seen as 10. As the boundary layer separates, a large low-pressure wake 11 is created behind the vehicle. Inside this wake the air is at a lower pressure than it is at the vehicle front. The pressure gradient across the vehicle produces a net force that acts to prevent movement of the vehicle—i.e., drag. This component of drag is called form drag.
The separation of the boundary layer flow of air over a ground vehicle is identical to the stall of an aircraft wing. When a wing is generating lift, the flow of air around the wing remains attached to the wing. See FIG. 2. As long as the flow remains attached, the form drag produced by the wing remains very small. FIG. 4 shows an airfoil 5 of chord 6. The wing's angle of attack 12 is the angle the chord 6 forms with the freestream 1. As the wing's angle of attack 12 is increased, a point will be reached where the boundary layer flow of air separates from the wing 10. The location where the flow separates is called the separation point 13. As a result of this separation a large wake 11 will be produced behind the wing. The wake 11 causes the drag produced by the wing to rapidly increase. In addition, the separation of the flow also produces a large reduction in lift generated by the wing. The reduction in lift is the result of large areas of the wing no longer being exposed to the boundary layer flow of air and thus being unable to produce lift. In order to avoid the consequences of boundary layer separation and wing stall, aircraft are routinely provided stall warning devices. Upon receiving an alarm from the stall warning device, a pilot will typically reduce the aircraft's angle of attack by pitching the nose down or rolling the wings level.
Airplane wings are highly streamlined and operate at very high speeds, especially when compared to the speed at which a ground vehicle typically travels. Wings are designed to operate with very little form drag because the boundary layer flow of air around the wing remains attached under all operating scenarios. An example of form drag on a non-streamlined object for illustrating ground vehicle performance is provided by a golf ball.
Most ground vehicles—particularly tractor trailer trucks—are bluff bodies, just like a golf ball. FIG. 5 shows a golf ball 14 exposed to a freestream flow of air 1. The separation of flow from the golf ball is seen as 13. As a result of this separation of flow, a large low pressure wake 11 is created behind the ball. The large pressure difference across the golf ball opposes the motion of the golf ball and produces drag. Unlike the operation of an airplane wing where separation must be avoided at all cost—hence the use of stall warning systems and highly streamlined shapes—it is a foregone conclusion that the flow of air around a golf ball will result in boundary layer separation. This is the result of the golf ball's non-streamlined shape. Once it is accepted that the boundary layer will separate from a golf ball in flight, the question becomes how to minimize the form drag resulting from the large low pressure wake 11 created by the boundary layer separation. The answer is to add dimples to the golf ball.
There are two types of boundary layer flow—laminar and turbulent. In laminar boundary layers, the flow is well ordered and all layers of flow are essentially parallel to each other. In contrast, turbulent flow is much more chaotic and unordered. There is also significant mixing between adjacent layers of flow in turbulent boundary layers. As a result of this mixing and the higher energy levels that exist in turbulent boundary layers, turbulent boundary layers will remain attached longer than laminar boundary layers. The dimples on a golf ball are designed to “trip” the boundary layer flow from laminar to turbulent. By tripping the boundary layer into a turbulent flow regime, separation of the boundary layer from the golf ball is delayed and form drag is reduced. The practical result of the reduced form drag is a golf ball that travels farther. The effect of adding dimples to a golf ball can be seen in FIG. 6, which shows a golf ball 14 again exposed to a freestream flow of air, 1. The golf ball contains dimples, 15. The dimples trip the boundary layer from laminar to turbulent. This increases the energy in the boundary layer and delays separation. Comparison of FIG. 6 to FIG. 5 shows that separation 13 now occurs later when the ball is dimpled. The delayed separation reduces the size of the low pressure wake 11 and allows the dimpled ball to travel further than the “non-dimpled” ball.
The distinction between skin friction drag resulting from the effects of viscosity in the boundary layer and form drag resulting from boundary layer separation and the creation of a low pressure wake along with the different techniques that can be utilized to minimize form drag have been known for some time. Indeed, Ludwig Prandtl—who is credited with identifying the boundary layer—studied the effect of suction on boundary layer separation as far back as 1904. More recently, several patent citations discuss active control of the boundary layer system in order to reduce vehicle drag.
U.S. Pat. No. 4,146,202 pertains to a porous aircraft skin that can be used with a suction source to maintain laminar flow over an aircraft wing. A system that utilized both suction and cooling via a cryogenic fluid to reduce aircraft drag is disclosed in U.S. Pat. No. 4,807,831. The cryogenic fluid can be provided from the aircraft's propulsion system—liquid hydrogen and liquid oxygen. As a result, it is not necessary to carry a cryogenic fluid for the sole purpose of working as part of the drag reduction system. Nevertheless, such a system would not be practical for a ground vehicle.
U.S. Pat. No. 5,222,698 is directed at a system that uses turbulence monitors to determine whether the boundary layer is laminar or turbulent. In the event turbulent flow is detected, suction from an external source can be applied to the boundary layer until laminar boundary layer flow is reestablished. The system is designed for highly streamlined aircraft structures—engine nacelles. Unlike the golf ball example cited above, boundary layer separation can be avoided with these highly streamlined structures. In the case of highly streamlined surfaces traveling at aircraft speeds, drag is minimized by keeping boundary layer flow laminar and thus reducing the skin friction drag.
U.S. Pat. No. 5,348,256 and U.S. Pat. No. 5,417,391 also describe systems that utilize active control of the boundary layer through suction. In particular, U.S. Pat. No. 5,348,256 suggests reducing noise on supersonic aircraft by allowing operation at higher angles of attack and reduced engine power levels. U.S. Pat. No. 5,417,391 discloses a series of vortex chambers acting in concert with a suction source to control the boundary layer flowing over an aircraft wing for the purposes of increasing the wing's lift/drag ratio. Like the other disclosures discussed above, these systems are designed for aircraft applications and do not identify the source of the suction.
U.S. Pat. No. 5,407,245 addresses the reduction of drag on a ground vehicle. Specifically, this patent disclosure identifies the “pressure resistance”—i.e. the creation of the low pressure wake behind a vehicle—as the chief contributor to vehicle drag. Indeed, it is disclosed that the form drag can be more than six times the skin friction drag. The low pressure wake is reduced utilizing a blower at the vehicle's rear to inject a high speed jet of air, which is estimated to require a speed of 50 m/s (approximately 110 miles/hour)—at the location where the boundary layer would otherwise separate from the vehicle. The high speed jet of air adds energy to the boundary layer and prevents boundary layer separation and the attendant increase in form drag that boundary layer separation produces.
In addition, by virtue of the vehicle's shape, the orientation of the various slots used to inject the high speed air, and the “Coanda effect,” a portion of the high energy air is also injected into the area of low pressure immediately behind the vehicle. U.S. Pat. No. 5,417,391 refers to this area as the “eddying zone,” and the resulting injection of high energy air into the eddying zone coupled with the suction of air from the eddying zone further reduces vehicle drag. The design calls for two blowers to provide both the injection of high speed air to prevent boundary layer separation and the suction to pull air through the eddying zone. This citation recognizes that any practical system must save more energy in the form of drag reduction than it requires in energy to power the blowers.
U.S. Pat. No. 5,908,217 uses another design to prevent boundary layer separation at the rear of the vehicle. In particular, the design uses a “source of compressed air” and control valves to regulate the injection of high speed air in the boundary layer at the vehicle's rear. A control system is utilized to direct the flow of air through several different plenums. By directing air through different plenums not only can the drag on the vehicle be reduced, but it is possible to control the pitching and yawing moments on the car as well as providing improved stability. Similarly, U.S. Pat. No. 6,068,328 proposes a boundary layer control system utilizing a series of “external perforation arrays and suction sources controlled by a digital signal processor.” The design utilizes a series of turbulence detectors to direct suction to those locations at risk of boundary layer separation.
Moreover, U.S. Pat. No. 7,810,867 suggests a vehicle wrapping product designed to fit over a vehicle's existing structure. The wrapping product is designed to induce turbulence in the boundary layer flowing over the vehicle. The “tripping” of the boundary layer from laminar flow to turbulent flow is intended to delay the onset of boundary layer separation and reduce the form drag. Essentially, the vehicle wrapping product performs a similar role to the dimples on a golf ball.